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Claims  |
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That which is claimed is:
1. A method for managing a population of metal parts in order to determine
when to remove an individual metal part from service, wherein said metal
parts are manufactured having relatively high levels of residual
compressive stress and said metal parts are subject to fatigue-related
failure, said method comprising:
(1) selecting an individual metal part from the population;
(2) determining the remaining residual compressive stress of the surface of
the selected individual metal part in one or more areas of stress
concentration using x-ray diffraction techniques; and
(3) removing the selected individual metal part from service if the
remaining residual compressive stress measured in the one or more areas of
stress concentration has fallen below a predetermined level.
2. A method as defined in claim 1, wherein the predetermined level of the
remaining residual compressive stress is about zero.
3. A method as defined in claim 1, wherein the predetermined level of the
remaining residual compressive stress is a fixed percentage less than 100
percent of the residual compressive stress in the part as manufactured.
4. A method as defined in claim 1, wherein the metal parts are rotating
parts used in a gas turbine or jet engine.
5. A method as defined in claim 4, wherein the metal parts are disks or
drum rotors.
6. A method as defined in claim 1, wherein the remaining residual
compressive stress measurements, generated over time, for the individual
parts in the population of parts are used to form a database that can be
used for predicting the service life of a given part in order to determine
whether, and when, that given part should be selected from the population
for determining the remaining residual compressive stress for that given
part.
7. A method as defined in claim 1, wherein the residual compressive stress
levels of the individual parts as manufactured are known and wherein the
residual compressive stress levels of the individual parts as manufactured
and the remaining residual compressive stress measurements, generated over
time, for the individual parts in the population of parts are used to form
a database.
8. A method as defined in claim 7, wherein the residual compressive stress
levels of the individual parts as manufactured are measured prior to, or
shortly after, the individual parts are initially placed into service.
9. A method as defined in claim 7, wherein said database is used to help
define or redefine specifications or design criteria for the individual
parts.
10. A method for managing a population of metals parts in order to extend
the service life of individual metal parts in the population, wherein said
metal parts are manufactured having relatively high levels of residual
compressive stress and said metal parts are subject to fatigue-related
failure during service, said method comprising, for each individual metal
part in the population:
(1) removing the individual metal part from the population from service;
(2) measuring the remaining residual compressive stress of the surface of
the individual metal part in one or more areas of stress concentration
using x-ray diffraction techniques;
(3) comparing the remaining residual compressive stress measured in the one
or more areas of stress concentration to a predetermined level; and
(4) if the remaining residual compressive stress remains above the
predetermined level, returning the individual metal part to service; or
(5) if the remaining residual compressive stress is at or below the
predetermined level, reworking the individual metal part to increase the
residual compressive stress to a reworked level above the predetermined
level and then returning the individual metal part to service.
11. A method as defined in claim 10, wherein steps (1) through (5) are
repeated periodically for the individual metal part until the individual
metal part must be permanently removed from service.
12. A method as defined in claim 10, wherein the predetermined level of the
remaining residual compressive stress is a fixed percentage less than 100
percent of the residual compressive stress in the part as manufactured.
13. A method as defined in claim 10, wherein the metal parts are rotating
parts used in a gas turbine or jet engine.
14. A method as defined in claim 13, wherein the metal parts are disks or
drum rotors.
15. A method as defined in claim 10, wherein the reworked level is at least
50 percent of the residual compressive stress level of the metal parts as
manufactured.
16. A method as defined in claim 14, wherein the reworked level is at least
80 percent of the residual compressive stress level of the metal parts as
manufactured.
17. A method as defined in claim 10, wherein the reworked level is about
the same as the residual compressive stress level of the metal parts as
manufactured.
18. A method as defined in claim 14, wherein the reworked level is about
the same as the residual compressive stress level of the metal parts as
manufactured.
19. A method as defined in claim 10, wherein the remaining residual
compressive stress measurements, generated over time, for the individual
parts in the population of parts are used to form a database that can be
used for predicting the service life of a given part in order to determine
whether, and when, that given part should be selected from the population
for determining the remaining residual compressive stress for that given
part.
20. A method as defined in claim 10, wherein the residual compressive
stress levels of the individual parts as manufactured are known and
wherein the residual compressive stress levels of the individual parts as
manufactured and the remaining residual compressive stress measurements,
generated over time, for the individual parts in the population of parts
are used to form a database.
21. A method as defined in claim 20, wherein the residual compressive
stress levels of the individual parts as manufactured are measured prior
to, or shortly after, the individual parts are initially placed into
service.
22. A method as defined in claim 20, wherein said database is used to help
define or redefine specifications or design criteria for the individual
parts.
23. A method for determining when to remove a metal part from service,
wherein said metal part is manufactured having relatively high levels of
residual compressive stress and said metal part is subject to
fatigue-related failure, said method comprising:
(1) measuring the remaining residual compressive stress of the surface of
the metal part in one or more areas of stress concentration using x-ray
diffraction techniques;
(2) comparing the remaining residual compressive stress measured in the one
or more areas of high stress concentration to a predetermined value; and
(3) removing the metal part from service if the remaining residual
compressive stress measured in the one or more areas of stress
concentration is less than the predetermined level.
24. A method as defined in claim 23, wherein the predetermined level of the
remaining residual compressive stress is about zero.
25. A method as defined in claim 23, wherein the predetermined level of the
remaining residual compressive stress is a fixed percentage less than 100
percent of the residual compressive stress in the metal part as
manufactured.
26. A method as defined in claim 23, wherein the metal part is a rotating
part used in a gas turbine or jet engine.
27. A method as defined in claim 26, wherein the metal part is a disk or
drum rotor.
28. A method for extending the service life of a metal part, wherein said
metal part is manufactured having relatively high levels of residual
compressive stress and said metal part is subject to fatigue-related
failure during service, said method comprising:
(1) measuring the remaining residual compressive stress of the surface of
the metal part in one or more areas of stress concentration using x-ray
diffraction techniques;
(2) comparing the remaining residual compressive stress measured in the one
or more areas of stress concentration to a predetermined level; and
(3) if the remaining residual compressive stress remains above the
predetermined level, returning the metal part to service; or
(4) if the remaining residual compressive stress is at or below the
predetermined level, reworking the metal part to increase the residual
compressive stress to a reworked level above the predetermined level and
then returning the metal part to service.
29. A method as defined in claim 28, wherein steps (1) through (4) are
repeated periodically for the metal part until the metal part must be
permanently removed from service.
30. A method as defined in claim 28, wherein the predetermined level of the
remaining residual compressive stress is a fixed percentage less than 100
percent of the residual compressive stress in the metal part as
manufactured.
31. A method as defined in claim 28, wherein the metal part is a rotating
part used in a gas turbine or jet engine.
32. A method as defined in claim 31, wherein the metal part is a disk or
drum rotor.
33. A method as defined in claim 28, wherein the reworked level is at least
50 percent of the residual compressive stress level of the metal part as
manufactured.
34. A method as defined in claim 32, wherein the reworked level is at least
80 percent of the residual compressive stress level of the metal part as
manufactured.
35. A method as defined in claim 28, wherein the reworked level is about
the same as the residual compressive stress level of the metal part as
manufactured.
36. A method as defined in claim 32, wherein the reworked level is about
the same as the residual compressive stress level of the metal part as
manufactured. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention generally relates to a method for managing the service life
of fatigue-limited metal components. This invention also generally relates
to a method for managing and extending the service life of fatigue-limited
metal components. More specifically, this invention is related to a
management method using a non-destructive technique for measuring the
remaining useful service life of fatigue-limited metal components by
determining the residual compressive stress in the critical surfaces of
the individual components. Using the method of this invention, a metal
component is either removed from service or reworked to increase its
residual compressive stress once the residual compressive stress is
reduced or falls below a predetermined value. This invention allows an
increase in both safety and economy in the management and operation of
turbine engines and other machines containing fatigue-limited metal
components by providing a reliable means for non-destructively measuring
the remaining service life of the fatigue-limited metal components. This
invention is especially adapted for managing populations of
fatigue-limited rotating parts in gas turbine engines, including aircraft
engines, and the like. Using the present methods of this invention to
manage and measure the residual compressive stress in such parts or
components, it is now possible to determine the appropriate time (i.e.,
prior to permanent deterioration from residual tensile stress crack
initiation) for reworking the part to increase or restore its residual
compressive stress so that the service life of the part or component can
be extended. Using the present methods of this invention, the overall
service life of a population of components used in, for example, jet
engines or turbine engines, can be maximized without a significant decease
in safety. In fact, the present invention can provide both increased
safety and economy for the aviation and other industries.
BACKGROUND OF THE INVENTION
Fatigue-limited metal components of gas turbines or jet engines, or other
machine components subject to metal failure or fatigue, must be carefully
managed in order to avoid failure during operation. The failure, for
example, of a critical component of a jet engine during operation may
result in loss of life or other catastrophic consequences. Currently in
the aviation industry (commercial and military), there are three general
types of management techniques or approaches used for the management of
fatigue-limited machine components in order to prevent possible
catastrophic failure due to metal fatigue. Each of these approaches
attempts to balance safety and economic concerns based on available data.
See, for example, S. Suresh, Fatigue of Metals, 499-502 (1991), which
generally discusses the three commonly used management approaches.
The most conservative of these approaches, often termed the "safe life"
approach, is based on the estimated fatigue life established through
analysis and comparable experience by the engine manufacturer. This
approach attempts to estimate the point at which the shortest-lived part
or component in the total population would be expected to fail. After
allowing for a suitable safety margin, an arbitrary retirement point is
adopted for that component. This retirement point is normally measured in
total take-off cycles or hours. Once a part reaches the retirement point,
it is removed from service and mutilated to prevent further, unauthorized
use. Although generally allowing for the greatest margin of safety,
significant economically useful service life of such parts is lost. In
effect, this "safe life" approach is based on, and controlled by, an
estimation of the lifetime of the weakest part or component in the total
population.
A somewhat less conservative management technique is the so-called "fail
safe" approach. In this approach, a maximum service life is determined by
the total accumulated service hours or cycles (whichever is shorter) at
which the first crack is detected in an actual part (i.e., disk or drum
rotor used in a compressor, turbine, or engine) in the population of like
parts. Once a part has developed such a crack, its accumulated service
life (in hours or cycles) is effectively used to determine the service
life of all similar parts in the population. If a part is later found to
develop a crack at an earlier time, then that part is then used to
redefine (and shorten) the acceptable service life limits of the
population. Once a part reaches its acceptable service life, it is removed
from service and mutilated to prevent further use. In effect, this "fail
safe" approach is also based on, and controlled by, the actual weakest
part or component in the total population. Many parts may still have many
hours of safe and useful service life remaining beyond that of this
weakest part. But, since the useful and safe service life of these parts
cannot be reliably determined, they must be removed from service in the
interest of safety. This "fail safe" approach is generally used in the
airline industry for mature fleets where low cycle fatigue cracks have
been detected in the relevant component populations. Where sufficient
service data has not been developed, the more conservative "safe life"
approach is generally used. In each approach, however, parts having many
remaining hours of safe and reliable use will be removed from service.
More recently, the United States Air Force has successfully adopted an even
less conservative management technique, the so-called "retirement for
cause" approach, for its management of some critical engine components. In
this approach, the parts are periodically examined non-destructively for
cracks using, for example, fluorescent dye penetration or magnaflux
techniques. Once a crack is observed, that part, but only that part, is
immediately retired from service. Other parts, even though they may have
accumulated service times equal to or greater than the retired part, are
continued to be used until they actually develop cracks. To operate
safely, this approach requires periodic and frequent inspections of the
individual parts. In general, as parts age, the frequency of inspections
should be increased. In any event, the frequency of inspections must be
such that the period between inspections is less, preferably by a
significant margin, than the time normally required for a detectable crack
to further deteriorate to the point of actual failure. Although this
approach may result in more frequent teardowns for inspection of the
individual parts, the potential savings based on achieving, or at least
approaching, the maximum lifetimes of the individual parts can be
enormous. The major drawback of this approach is that it relies upon
detection of an actual crack in the part. Thus, this approach is generally
not suitable for parts in which crack formation cannot be detected in a
reliable and consistent manner. Once a crack has formed, the part
contains, in effect, a permanent, irreversible defect which will
ultimately lead to failure, perhaps catastrophic failure, unless that part
is removed from service in a timely manner. Additionally, this approach,
of course, is not suitable for use where the normal time between the
initial development of a detectable crack and failure of the part is
relatively short. Moreover, in parts where actual failure normally does
not follow quickly after the development of a crack, if such a crack
develops shortly after an inspection, the risk of failure during actual
operation increases simply because the length of time in which the part is
operated with the defect is maximized. Therefore, this method has an
increased safety risk when compared to the "safe life" and "fail safe"
approaches. This increased risk, although perhaps small, may still be
significant because the detection point is the actual formation of a
detectable crack. The longer that part remains in service, once a crack
has formed, the greater the risk of catastrophic failure.
It would be desirable, therefore, to provide non-destructive methods to
measure the remaining service or useful life of fatigue-limited metal
components before crack initiation has begun or, at least, before actual
cracks can be observed (i.e., before permanent and irreversible damage has
begun). It would also be desirable to provide methods by which the service
or useful life of fatigue-limited metal components could be increased
without significantly increasing the risk of catastrophic failure of the
metal components during operation. Such methods would provide both
increased safety and economy for the aviation industry (commercial and
military). The methods of this invention generally provide such improved
methods.
SUMMARY OF THE INVENTION
The present invention relates to methods for the management of populations
of fatigue-limited metal components. This invention also relates to
methods for the detection of the remaining service life of individual
fatigue-limited metal components. The metal components to be managed by
the present invention include metal components having relatively high
levels of residual compressive stress as manufactured and which are
subject to fatigue-related failure. The relatively high residual
compressive stress of such a metal component as manufactured may be the
result of the actual manufacture process used and/or subsequent working of
the metal component by shot peening or other cold working processes after
actual production to increase the residual compressive stress. Preferably,
the residual compressive stress as manufactured is in the range of about
50,000 to 200,000 pounds per square inch and, more preferably, in the
range of about 150,000 to 180,000 pounds per square inch. Components
having residual compressive stresses higher or lower than these ranges
can, of course, be managed by the methods of the present invention.
However, the components as manufactured should have sufficient residual
compressive stress for their intended use. Using the methods of this
invention, increases in both safety and economy in the management of such
metal components is expected.
Fatigue failures in metal components almost always develop from cracks
generated in the surface layer of the metal components exposed to high
stress environments. To reduce the likelihood of crack formation, great
care is normally taken in the manufacture of such metal components to
ensure that the initial residual stress in the critical surface layers of
the crystalline structure of the metal are in relatively high compression
(often up to 170,000 pounds per square inch or higher). During operation
under conditions of high load and operating temperatures, the residual
compressive stress of the component gradually diminishes over time. Once
the residual compressive stress reaches zero, the trend continues and
builds up residual tensile stress in these areas. Over time, the residual
tensile stress can increase to levels in excess of the ultimate tensile
strength of the surface of the material and cracks develop. Such cracks in
a component left in service propagate until they reach a critical length,
at which time catastrophic failure occurs. The present invention provides
methods for managing metal components whereby conditions involving
significant residual tensile stress and, therefore, crack initiation are
avoided. By monitoring the residual compressive stress in areas of high
stress concentration and maintaining the metal component under conditions
of compressive stress, the present invention provides a management program
which does not rely on either expected or actual crack formation as the
management criteria.
In the method of this invention, a non-destructive technique (i.e., x-ray
diffraction) is used to measure the remaining residual compressive stress
in the relevant metal components. Once the residual compressive stress of
an individual component falls below a predetermined value, that part, but
only that part, is effectively flagged for further attention. For metal
components having residual compressive stress below a predetermined value,
there are essentially two options. In the first option, the metal
components is simply removed permanently from service. In the second
option, the metal component is reworked (using, for example, shot peening)
to increase its residual compressive stress and then returned to service.
By periodically evaluating such metal components using the methods of this
invention, the service life of the total population of metal components
can be maximized in a safe and efficient manner.
For metal components having residual compressive stress higher than the
predetermined value, the remaining service life of that component can be
determined. The greater the difference between the measured residual
compressive stress and the predetermined value, the greater the remaining
service life for that component should be. Such information should be
useful (especially as considerable historical data for the population
becomes available over time) in matching components for use in particular
engines or applications (i.e., matching components which have comparable
remaining service life) or for scheduling routine teardowns and
maintenance.
One object of the present invention is to provide a method for managing a
population of metal parts in order to determine when to remove an
individual metal part from service, wherein said metal parts are
manufactured having relatively high levels of residual compressive stress
and said metal parts are subject to fatigue-related failure, said method
comprising:
(1) selecting an individual metal part from the population;
(2) determining the remaining residual compressive stress of the surface of
the selected individual metal part in one or more areas of stress
concentration using x-ray diffraction techniques; and
(3) removing the selected individual metal part from service if the
remaining residual compressive stress measured in one or more areas of
stress concentration has fallen below a predetermined level.
Another object of the present invention is to provide a method for managing
a population of metals parts in order to extend the service life of
individual metal parts in the population, wherein said metal parts are
manufactured having relatively high levels of residual compressive stress
and said metal parts are subject to fatigue-related failure during
service, said method comprising, for each individual metal part in the
population:
(1) removing the individual metal part from the population from service;
(2) measuring the remaining residual compressive stress of the surface of
the individual metal part in one or more areas of stress concentration
using x-ray diffraction techniques;
(3) comparing the remaining residual compressive stress measured in one or
more areas of stress concentration to a predetermined level; and
(4) if the remaining residual compressive stress remains above the
predetermined level, returning the individual metal part to service; or
(5) if the remaining residual compressive stress is at or below the
predetermined level, reworking the individual metal part to increase the
residual compressive stress to a reworked level above the predetermined
level and then returning the individual metal part to service.
Although the present invention is preferably directed towards methods for
the management of large populations of similar type metal parts, it can
also be used to test individual metal parts. Thus, for example, the
present invention can also be used for spot checking metal parts
throughout their expected service life as part of routine or scheduled
preventive maintenance or during repairs or teardown procedures
necessitated by breakdowns. Thus, still another object of the present
invention is to provide a method for determining when to remove a metal
part from service, wherein said metal part is manufactured having
relatively high levels of residual compressive stress and said metal part
is subject to fatigue-related failure, said method comprising:
(1) measuring the remaining residual compressive stress of the surface of
the metal part in one or more areas of stress concentration using x-ray
diffraction techniques;
(2) comparing the remaining residual compressive stress measured in one or
more areas of high stress concentration to a predetermined value; and
(3) removing the metal part from service if the remaining residual
compressive stress measured in one or more areas of stress concentration
is less than the predetermined level.
Still another object of the present invention is to provide a method for
extending the service life of a metal part, wherein said metal part is
manufactured having relatively high levels of residual compressive stress
and said metal part is subject to fatigue-related failure during service,
said method comprising:
(1) measuring the remaining residual compressive stress of the surface of
the metal part in one or more areas of stress concentration using x-ray
diffraction techniques;
(2) comparing the remaining residual compressive stress measured in one or
more areas of stress concentration to a predetermined level; and
(3) if the remaining residual compressive stress remains above the
predetermined level, returning the metal part to service; or
(4) if the remaining residual compressive stress is at or below the
predetermined level, reworking the metal part to increase the residual
compressive stress to a reworked level above the predetermined level and
then returning the metal part to service.
These and other objects and advantages of the present invention will be
apparent from a consideration of the present specification and drawing.
DESCRIPTION OF FIGURES
FIG. 1 illustrates a typical disk from a jet engine showing areas of stress
concentration in which residual compressive stress should be determined.
FIG. 2 shows a typical plot of residual compressive stress as a function of
service hours for turbine disks operating under different load and
temperature conditions.
DETAILED DESCRIPTION OF THE INVENTION
The invention generally relates to methods for managing the service life of
fatigue-limited metal components and to methods for managing and extending
the service life of fatigue-limited metal components. The methods of this
invention employ a non-destructive technique for measuring the remaining
useful service life of fatigue-limited metal components by determining the
residual compressive stress in the critical surfaces of the individual
component. The residual compressive stress can be correlated with the
remaining service life of the individual component. If the residual
compressive stress has not fallen below a predetermined value, the
component can be returned to service. If the residual compressive stress
reaches or falls below a predetermined value, the component can be
permanently removed from service. Or, if desired and appropriate, the
component can be reworked to increase the residual compressive stress to a
level above the predetermined value, preferably a value approaching the
compressive stress of the component as originally manufactured, and then
returned to service.
It is well established that fatigue failures develop from cracks generated
in the surface layer of metal components exposed to high stress
environments. For example, failure normally occurs because of cracks
forming in areas of stress concentration in such components. FIG. 1
illustrates such a component, specifically a disk 10 for use in a gas
turbine engine. Failure of such a disk is often caused by cracks forming
in the surface layers of areas of high stress concentration such as the
inside radii or bottom 12 of dovetail or firtree slots 14. These dovetail
or firtree slots are used for attachment of the compressor and turbine
blades (not shown). To reduce the likelihood of crack formation, great
care is normally taken in the manufacture of such components to ensure
that the initial residual stress in the critical surface layers of the
crystalline structure of the metal are in high compression. For example,
turbine disks, such as the one illustrated in FIG. 1, are generally
manufactured with residual compressive stress in the order of about
170,000 pounds per square inch. During operation in a turbine engine
(i.e., conditions of high load and operating temperatures), the residual
compressive stress gradually diminishes over time as shown in FIG. 2. The
curves in FIG. 2 (labeled 20, 22, 24, 26, and 28) are for different
turbine disks 10 (i.e., different stages) used in a gas turbine engine. In
such an engine, each disk is subjected to different load and temperature
conditions during operation. Thus, the rate of decrease of the residual
compressive stress is different for each disk or stage. Once the residual
compressive stress reaches zero, residual tensile stress can build up in
these areas. Over time, the residual tensile stress can increase to levels
in excess of the ultimate strength of the surface of the material and
cracks will initiate. Such cracks in a component left in service propagate
until they reach a critical length, at which time catastrophic failure
will occur.
The methods of this invention monitor the residual compressive stress in
areas of stress concentration in order to prevent the initial formation of
cracks in the surface. By removing the component from service before the
stresses change from compressive to tensile in nature or by maintaining
the stress in compression, the present methods allow individual components
to be used in a manner in which surface cracks are not formed or are, at
least, formed at a significantly lower rate as compared to current
management methods. The present methods allow for achievement of the
maximum service life of components without increased failure or safety
risks. Moreover, the present methods allow for significantly extending the
service life of individual components without increased failure or safety
risks.
In practice, for a given type and population of metal components, the
residual compressive stress in the surface layer of the individual
component is measured in one or more areas of stress concentration using
x-ray diffraction techniques. The actual area measured is normally in the
range of about 1/4 by 1/4 inches to about 1 by 1 inch, although smaller or
larger areas can be used if desired. The measured value (or values or
averaged value) is compared to a predetermined value. If the measured
value is above the predetermined value, the part can be returned to
service. If however, the measured value equals or falls below this
predetermined value, the component can be treated in several ways. In a
first option, the component can be permanently removed from service. In
such cases, it is generally preferred that the component be mutilated, or
otherwise marked, to prevent further, unauthorized use. In a second
option, the component can be reworked to increase its residual compressive
stress and then returned to service. Normally, such a component can be
reworked and returned to service a fixed number of times or cycles (i.e.,
until other failure mechanisms predominate or the component no longer
meets design criteria or specifications). The acceptable number of cycles
for reworking such components will generally be determined on a
case-by-case basis. For a given component, such as the disk shown in FIG.
1, the cycle of service and reworking can generally be repeated so long as
the component retains its dimensional and microstructural stability.
In some cases when the residual compressive stress is above, but close to
or approaching, the predetermined value, it may be preferred to rework
that part at that time rather than wait for the residual compression
stress to fall below the predetermined value. For example, if the measured
residual compressive stress suggests that the part has only a relatively
short service life remaining before reworking will be required (see, for
example, curve 22 in FIG. 2 at 15,000 hours with a predetermined value of
zero pounds per square inch), it may be more economical to rework the part
during the current scheduled shutdown/teardown event rather that put the
part back in service and then require an unscheduled teardown to rework it
only a short time later. Whether this modified approach is appropriate and
desirable in a given case will depend, in larger part, on the expected
service life before that part will fall below the predetermined value. If
the expected remaining service life is short (thereby necessitating a
unscheduled teardown for remeasurement), it may be more economical to
rework that part even though it has remaining service life. In such a
case, the predetermined value is effectively increased for that part only.
As one skilled in the art will realize, the predetermined value of the
residual compressive stress for any given population of components will
depend, at least in part, on the residual compressive stress of the
components as originally manufactured, the specific physical and
metallurgical characteristics of the components, the environment in which
the components are used, and any appropriate safety factors. For
populations of different parts, this predetermined value will likely be
different because of different designs of the parts and exposure to
different stresses during use. Populations of the same parts, but operated
under different conditions and environments, may also have different
predetermined values. Moreover, for a given component, different areas of
stress concentration may have different predetermined values. For example,
different areas of metal component will normally be exposed to, or will
experience, different levels of stress and may, therefore, experience
changes in the compressive stress at different rates. In such cases, the
area that reaches its predetermined value first will normally control the
disposition of that component. Although not necessary, it will generally
be preferred that the initial compressive residual stress of the metal
components be measured or otherwise known before they are placed in
service, or shortly thereafter. Measurement of the residual compressive
stress of a component as originally manufactured can help insure that only
components meeting specifications are used and can provide benchmarks for
later measurements of remaining residual compressive stress. Moreover,
such initial residual compressive stress data, along with the data
generated by the present methods, can be used to define or redefine
component specifications and design criteria as appropriate.
The predetermined value can be expressed in terms of absolute numbers
(e.g., a specific value in suitable units for the residual compressive
stress) or in relative numbers (e.g., a percentage of the remaining
compressive stress of the component as compared to the residual
compressive stress as originally manufactured). Moreover, the
predetermined value for a given population may change over time as more
historical data becomes available. For example, for newly designed
components, it may be desirable to use a relatively high predetermined
value to guard against unexpected failures for increased safety. As
service life data becomes available, however, it may be appropriate to
decrease the predetermined value if significant safety or failure related
problems are not found in the population. By carefully adjusting the
predetermined value for a given population of components over time, it
should be possible to approach the optimum value while maintaining
operational safety.
In some cases, a predetermined value of about zero pounds per square inch
(or other appropriate units) or 100 percent decrease in residual
compressive stress (i.e., the point at which stress moves from compressive
to tensile in nature) may be appropriate. The use of zero pounds per
square inch or 100 percent decrease as the management criteria might, for
example, be appropriate to maximize the service life of a component where
reworking the component is not practical or is otherwise not anticipated.
In most cases, however, a predetermined value of a value greater than zero
pounds per square inch or less than 100 percent decrease will generally be
preferred and appropriate based on safety considerations. Such higher
predetermined values will be especially preferred where reworking of the
component to restore all or part of the residual compressive stress is
anticipated. In some cases, however, a predetermined value of less than
zero pounds per square inch or greater than 100 percent decrease may be
appropriate.
As noted above, the residual compressive stress is measured
non-destructively using conventional x-ray diffraction techniques.
Preferably, the residual compressive stress is measured using portable
x-ray diffraction equipment. Examples of such x-ray equipment and
techniques can be found in U.S. Pat. No. 5,125,016 (Jun. 12, 1992); Taira
& Tanaka, "Residual Stress Near Fatigue Crack Tips," 19 Transactions of
the Iron & Steel Institute of Japan, 411-18 (1979); Harting & Fritsch, "A
Non-destructive Method to Determine the Depth-dependence of
Three-dimensional Residual Stress States by X-ray Diffraction," 26 J.
Phys. D: Appl. Phys., 1814-16 (1993); Kuhn et al., "An X-ray Study of
Creep-deformation Induced Changes of the Lattice Mismatch in
.gamma.'-Hardened Monocrystalline Nickel-Base Superalloy SRR 99," 39 Acta
Metall. Mater., 2783-94 (1991), all of which are hereby incorporated by
reference. Portable x-ray equipment, which is generally preferred in the
present invention, can be obtained commercially from, for example,
Technology for Energy Corporation (P.O. Box 22996, Lexington Drive,
Knoxville, Tenn. 37933) or American Stress Technologies, Inc. (61 McMurray
Road, Pittsburgh, Pa. 15241). Other types and designs of x-ray diffraction
equipment or techniques can also be used in the present invention.
Normally such measurements should be made, at a minimum, during scheduled
teardowns and other maintenance events. In some cases, however, it may be
desirable to make such measurements more often than regularly scheduled
maintenance events, especially during the early service life of a
population of newly designed components lacking a extensive service life
history. Normally, such measurements of the residual compressive stress
will be made on the individual parts during teardowns. For some
components, however, it may be possible to make the necessary measurements
without having to perform complete teardowns. As noted above, x-ray
diffraction measurements of residual compressive stress should be made in
areas of high stress concentration (e.g., the bottom 12 of the firtree
slots 14 on the disk 10 shown in FIG. 1). Generally, areas of high stress
concentration are those areas in which crack formation has been observed
or is more likely to occur. It is not necessary, however, to make such
measurements in each and every area of high stress concentration in a
given component, especially where such areas are operated under similar
load and temperature conditions. For the disk in FIG. 1, for example,
measurements might be taken on the bottom 12 of the firtree slots 14
located at 0, 90, 180, and 270 degrees, rather than at the bottom of every
slot 14. The individual measurements at these representative locations, or
an average of the individual measurements, are compared to the
predetermined value. As the database develops, the number and location of
measurements for a given disk (or other population of components) can be
modified as appropriate.
Once a component reaches or falls below its predetermined value, it can
either be removed permanently from service or reworked to increase its
residual compressive stress to a level above the predetermined value and
then returned to service. For example, using a predetermined value of zero
pounds per square inch, the disk represented by curve 28 in FIG. 2 should
be removed from service or reworked after about 10,000 hours of service;
the disks represented by curves 24 and 26 should be removed from service
or reworked after about 15,000 hours; the disks represented by curves 20
(especially) and 22 (to a lesser extent) have service lives greater than
15,000 hours. Preferably, the residual compressive stress in such reworked
components is returned to a level approaching the original residual
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